An exemplary motor drive system includes the following main components: a synchronous motor, such as a three-phase permanent magnet synchronous motor (PMSM); a multi-phase power converter; a DC power source; a PWM (Pulse Width Modulation) Generator and switch driver; and a controller. The power converter is typically an inverter that converts DC power from power source into three-phase AC power, e.g., utilizing a configuration of insulated-gate bipolar transistors (IGBTs) under control of the PWM Generator and switch driver (pulse width modulation (PWM) control).
The controller controls the power inverter via the PWM Generator and switch driver so that the power converter outputs the desired multi-phase AC power to the stator windings of motor. Thus, during operation of the synchronous motor, the power converter converts DC power from the power source into multi-phase AC power and supplies such multi-phase AC power to stator windings of the motor, creating a rotating magnetic field that interacts with the rotor's magnetic field to create torque. Thus, proper control of the power converter during normal operation, as a function of rotor position/speed, is necessary to generate a rotating magnetic field that results in efficient motor function, particularly for a variable speed drive system.
A synchronous AC motor drive system of the type described about typically utilizes rotor position sensors to provide information regarding the position and speed of the motor's rotor with respect to the motor's stator windings. Such positional information allows for proper conversion of power that is supplied to the stator windings. Rotor position sensors, however, can be unreliable due to mechanism alignment and temperature incompatibility problems. Moreover, the rotor position sensors can be difficult to mount to the motor during motor assembly, especially for multi-pole motors.
Motor drive systems without rotor position sensors (“speed sensorless” motor drive systems) have become increasingly popular in industrial and aerospace applications due to their low cost and high reliability operation, especially at high speed. Some of the sensorless algorithms, such as Instantaneous Power-Floating Frame Control described in U.S. patent application Ser. No. 10/862,960, filed Jun. 8, 2004, estimate rotor position based on the output voltage of the current loop proportional integral (PI) regulators instead of using measured motor terminal voltage and current signals, which makes the estimated speed more robust to the measurement noise.
Both speed sensorless and sensor-based motor drive systems should respond to fault conditions, such as motor over speed, motor over current and converter DC bus over voltage, . . . etc., even when such fault conditions are transient. Typically, electrical contactors between the power converter and the motor are opened and IGBT gating is disabled upon detecting such non-critical fault conditions, thereby causing the motor rotor to decelerate. If the fault is cleared and the motor rotor still has sufficient speed, the motor drive system will attempt to rerun (i.e., a flying run), which requires closing the electrical contactors, enabling IGBT gating, and resynchronizing. If the motor speed drops below a certain level before the fault is cleared, complete shut-down is required before restart. A new restart will require to go through a full “soft-start” process.
In a speed sensorless system, system operation is necessary to derive motor position/speed information, which makes it difficult to achieve fault tolerance control and flying run operations. Because such systems will lose rotor position information, it is difficult to achieve resynchronization after a temporary shut down.